Can you tell who is lying?

Mr. John lives in a mansion. He is very rich.
He is so rich that he has a maid, a cook, a butler, and a lifeguard.
Yesterday, Mr. John was found dead. The chief detective was coming from Sunday church to investigate. The police were already there. They caught six suspects. The maid, the butler, the cook, the lifeguard, the postman, and Mrs. John.
The maid said she was dusting. The butler said he was playing a board game with Mr. John’s sons.
The cook said he was making dinner. The lifeguard said he was watching Mr. John’s daughter swim in the pool.
The postman said he was delivering mail. Mrs. John said she was watching TV.
Can you tell who is lying?